Evaluation on fatigue cracking resistance of fiber grid reinforced asphalt concrete with reflection cracking rate computation

Construction and Building Materials 239 (2020) 117873

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Evaluation on fatigue cracking resistance of fiber grid reinforced asphalt concrete with reflection cracking rate computation Allistair Bliss Tam a, Dae-Wook Park b,⇑, Tri Ho Minh Le c, Jo-Soon Kim d a

Dept. of Civil Engineering, Kunsan National University, 558 Daehak Ro, Kunsan, Jeonbuk, Republic of Korea Dept. of Civil Engineering, Kunsan National University, 558 Daehak Ro, Kunsan, Jeonbuk, Republic of Korea c Dept. of Civil Engineering, Kunsan National University, 558 Daehak Ro, Kunsan, Jeonbuk, Republic of Korea d SN Construction Co, Ltd, 1708 Dongseo Dero, Dong gu, Daejeon, Republic of Korea b

h i g h l i g h t s
The OT test using cyclic fatigue loading was able to delineate fiber grid materials.
Carbon fiber grids perform best in improving fatigue cracking resistance.
Crack rate index makes a good surrogate interpretation for OT test.

a r t i c l e

i n f o

Article history: Received 5 September 2019 Received in revised form 1 November 2019 Accepted 13 December 2019

Keywords: Overlay Tester Fiber grid reinforcement Reflection cracking rate Critical fracture energy Crack rate index

a b s t r a c t Fiber grid reinforcement typically used in rehabilitated pavement has shown to mitigate reflective cracking. In this study, different fiber grid materials were evaluated based on fatigue cracking resistance using the Overlay Tester (OT) Machine. A comparison on the number of cycles to failure and a surrogate method was conducted. This observation led to further analysis on three sample configurations to evaluate its performance year based on reflection cracking rate. The test results indicated that using fiber grid reinforcement substantially increase the fatigue cracking resistance. Good correlation was observed between crack rate index and number of cycles to failure. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The performance life of an asphalt overlay shortens drastically when the old pavement for rehabilitation shows severe cracking. The primary reason is the propagation of cracks from the old pavement onto the surface. This behavior on the rehabilitated pavement is called reflection cracking. This type of distress is most commonly found on pavements with low load transfer efficiencies on joints [1]. The inability of joints to move as a single entity causes shear stress along the normal plane during traffic loadings. Likewise, with the added temperature fluctuations resulting to contraction or expansion on the pavement, adds to the reflection cracking rate (RCR) phenomenon. It has been successfully recorded that the use of fiber grid reinforcement extends service life by reducing reflection cracking on asphalt overlays [2–7]. Studies have shown that the presence of a ⇑ Corresponding author. E-mail address: [email protected] (D.-W. Park). https://doi.org/10.1016/j.conbuildmat.2019.117873 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

fiber grid reinforcing material does not essentially affect during crack initiation but rather during crack propagation [2,8,9]. Laboratory research conducted by Gonzalez-Torre [10] showed that among the different types of asphalt reinforcement, fiber grids reduced cracking the most. In addition, it was concluded that grid reinforcement had higher structural superiority compared to nonwoven reinforcement due to the presence of adequate apertures which influence adherence between pavement layers. Moreover, a model mobile load simulator (MMLS) was used by Kim [11] to investigate the effects of grid reinforcement on asphalt concrete slabs. Using carbon fiber reinforcement polymer (FRP) grids showed a significant reduction during wet condition testing on surface deflection and fatigue life by 48% and 27%, respectively [11]. The study of Zofka [3,8] on reinforced asphalt concrete beams using glass and carbon fiber grid also agrees well with most studies showing substantial improvement from cyclic and monotonic loadings. Based on post-peak fracture energy, using carbon fiber grids was observed to obtain four times the energy compared to unreinforced samples. Khodaii [5] showed the effects of

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

reinforcement on the reduction of reflection cracking in asphalt overlays. The study concluded that specimens with embedded grid reinforcement out-performed unreinforced samples both in terms of cracking resistance and rutting. The study to quantify the benefits of grid reinforcement has not only been limited to empirical methods. The current asphalt overlay design method proposed by Zofka [12] showed a practical procedure on accounting the effect of geogrid by incorporating both empirical and mechanistic elements together. The novel method was capable of acquiring the service life of an asphalt overlay given the input parameters such as traffic, degree of existing pavement condition, and asphalt overlay geogrid parameters. With the presented new approach, there still exist areas for continued improvement on the interpretation of the beneficial influence of fiber grid reinforcing materials. Even though there are numerous benefiting results presented on the use of fiber grid reinforcing materials, the regulation on the testing procedure is still currently undecided. With no recognized standard, the test method falls short on being widely implemented. On the other hand, a potential method with extensive research history is presented using the Overlay Tester (OT) machine from Texas A&M Transportation Institute (TTI) [13]. It quantifies fracture characteristic by inducing a dynamic or constant displacement movement perpendicular to the axis of crack propagation. In addition, it has successfully evaluated both laboratory and field samples [6,14]. Primarily designed to study asphalt overlay cracking behavior, recent report show that the use of OT machine can also distinguish reinforcing materials used on an asphalt concrete sample [14]. A study by Walubita [15] has shown the capability of using the OT machine in characterizing cracking and fracture properties using monotonic loading test. At present, fiber grids used in Korea as pavement reinforcement has been tested mostly only to characterize its physical property. However, this does not capture the influence of the reinforcing material once installed in an asphalt overlay. To safeguard quality and improve material selection, this study employs the OT machine in evaluating different fiber grid reinforcing materials. From the OT test, this study aims to use cyclic loading to characterize the fatigue cracking resistance of fiber grid reinforced specimen based on the number of cycles to failure. Moreover, fracture energy

a.

from the hysteresis loop and the power coefficient of load reduction curve was obtained for comparison. Further analysis was instigated by developing an automated program that calculates pavement performance year based on reflection cracking model following TTI and the Mechanistic-Empirical Pavement Design Guide (MEPDG) procedures. 2. Materials and specimen preparation 2.1. Experimental plan In this research, five different fiber grid reinforcing materials were utilized primarily made of glass fiber filaments denoted as FG1, FG2, FG3, FG4, and FG5. These fiber grid reinforcing materials were chosen based on local market availability. Among the fiber grids, only FG5 grid differs by including carbon fiber filaments along cross machine direction (CMD). Carbon fiber filaments were used specifically to improve grid tensile strength while reducing overall weight. Fig. 1a shows the proper terms used in defining the fiber grid reinforcing material in this study. Furthermore, two types of asphalt mixes were used including dense grade and polymer modified stone mastic asphalt (PSMA). Table 1 shows the summary of each sample configuration, prepared with a minimum of 3 replicates each. As shown in Table 1, all samples include two layers with a total thickness of 60 mm. The reinforcing grid material was placed in between layers with tack coat on all conditions. As depicted, sample configuration #1 without any fiber grid reinforcement is defined as the control sample. The control sample uses PSMA mix for both upper and lower layers, while samples #2 to #5 utilize a dense grade lower layer. The reason behind the configuration on the latter samples is to replicate the widely encountered old dense grade pavements at the actual site. In addition, samples #6 and #7 uses the same PSMA mix with control sample to investigate the influence of fiber grid material when all conditions are considered identical. 2.2. Description on fiber grids and asphalt mixes For sample preparation, this experiment used different materials including 2 types of asphalt concrete mixes and 5 different fiber

b.

c.

e.

f.

Aperture

Rib

d.

MD

CMD

Junction

Fig. 1. Showing (a) Grid terminology, (b) FG1 grid, (c) FG2 grid, (d) FG3 grid, (e) FG4 grid, and (f) FG5 grid.

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873 Table 1 Summary on test sample configuration. Description

Thickness

Upper layer Grid Tack coat Lower layer

30 mm 1 mm n/a 30 mm

Sample #1

#2

#3

#4

#5

#6

#7

PSMA No Yes PSMA

PSMA FG-1 Yes Dense

PSMA FG-2 Yes Dense

PSMA FG-3 Yes Dense

PSMA FG-4 Yes Dense

PSMA FG-4 Yes PSMA

PSMA FG-5 Yes PSMA

Table 2 Aggregate gradation. Sieve size (mm)

Dense Grade

PSMA mix

Limits

19 13.2 9.5 4.75 2.36 0.6 0.3 0.15 0.075

Used

Min.

Max

100 90.0 73.0 40.0 25.0 11.0 7.0 4.0 3.0

100 100 90.0 60.0 40.0 22.0 16.0 12.0 9.0

grid reinforcing materials. The properties and description of each material are discussed in the following sections. 2.2.1. Asphalt concrete and tack coat The lower layer of the test specimen included two types of asphalt concrete mixes. This consisted generally of bitumen binder, aggregates, and filler material. The mixtures were mixed thoroughly in an asphalt mixing plant and transported to the laboratory for the research purpose. The aggregate gradation for both mixes is shown in Table 2. The first type of asphalt mix was a dense grade with a nominal maximum aggregate size (NMAS) of 13 mm, using PG64-22 binder, and a maximum specific gravity of 2.467. While the second mixture was a PSMA mix with an NMAS of 13 mm using PG76-22 binder and a maximum specific gravity of 2.466. Zofka [17] expressed the study of interlayer bonding and shear resistance as the key elements on pavement performance. The presence of a reinforcing material between layers may weaken the interface shear strength. For layers to adhere adequately, a modified asphalt emulsion tack coat was used in this experiment having an interlayer shear strength (ISS) value of 962 kPa in accordance with Tex-249-F [18]. From National Cooperative Highway Research Program (NCHRP) report 712, it is stated that an ISS value of 276 kPa is adequate for asphalt overlays considering a safety factor of 1.4 against variability in measurements and in construction [19]. In addition, the tack coat property includes ductility, softening point, and penetration value of 100 + cm, 61.5 °C, and 68, respectively. 2.2.2. Bare biaxial grid, FG1 As shown in Fig. 1b, FG1 grid presents a bare structure with glass fiber filaments weaved into a grid formation. This grid forms a square aperture size of 15 mm, and a rib width of an approximate 4 mm. The rib along machine direction (MD) has an intertwined string to prevent movement of CMD ribs. Rib thickness ranges from 0.35 mm to 0.47 mm at junction. In addition, FG1 grid was reported to have a tensile strength of 50 kN/m. Due to the absence of any film or coating, the grid is hard to work with and can distort effortlessly. For testing, the movement of the cyclic load is directed parallel to machine direction.

100 100 81.4 51.1 33.5 18.0 11.0 9.2 7.5

Limits

Used

Min.

Max

100 93.0 40.0 16.0 12.0 10.0 8.0 7.0 7.0

100 100 55.0 30.0 23.0 18.0 15.0 14.0 12.0

100 100 43.2 27.5 20.5 14.0 12.8 10.1 8.3

2.2.3. Bare biaxial grid with PE film, FG2 This composite grid comprises of bare glass fiber filaments attached to a polyethylene (PE) film that helps support the grid formation. Similar to FG1 grid, the rib along MD was held together with a string. An actual photo of FG2 grid is shown in Fig. 1c. An aperture size of 10 mm (MD)  20 mm (CMD) was observed, having a thickness of 0.95 mm to 1.05 mm at junction. In addition, rib width was measured at an average of 4.5 mm and having a 60 kN/m tensile strength. Grid installation requires the film oriented upwards, while cyclic loading was tested along the machine direction. 2.2.4. Coated biaxial grid with EVA film, FG3 A bituminized coated biaxial glass fiber grid attached to an ethylene vinyl acetate (EVA) film, forming a square aperture size of 15 mm and with an approximately 1.6 mm thickness as shown in Fig. 1d. The rib width was measured at an average of 4.5 mm and having a 50 kN/m tensile strength. It was observed that the grid product has two sides, including the smooth bitumen surface attached to the film and secondly, the other side with sprayed quartzite sand. It was instructed to orient the film facing upwards. In testing FG3 grid reinforced asphalt samples, the movement of the cyclic loading is directed parallel to the fiber grids on either direction. 2.2.5. Coated biaxial grid with PP film, FG4 FG4 grid portrays the same physical characteristics to FG3 grid with an exception on the type of film used. The interwoven coated glass fiber filaments had a thin polypropylene (PP) film backing, it had a square aperture size of 15 mm and an approximate thickness of 1.5 mm as shown in Fig. 1e. The rib width was measured at an average of 5.25 mm while having a 50 kN/m tensile strength. The grid product has two sides similar to FG3 grids, having a clear thin PP film at one side and sprayed quartzite sand on the other. In installing FG4 grids, the orientation of the sprayed quartzite sand faces upward, while the movement of the cyclic loading can go either direction as long as parallel to the grid. 2.2.6. Coated biaxial grid with carbon fiber and PP film, FG5 High strength and high tenacity bituminized coated filaments using glass fiber along MD and carbon fiber along CMD. The grid

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portrays a square aperture of 15 mm having a rib thickness of about 1.7 mm as shown in Fig. 1f. The rib width was measured at an average of 5.0 mm, having a tensile strength of 50 kN/m and 80 kN/m for MD and CMD, respectively. FG5 grid also shows similar face to FG4 grid with a sprayed quartzite sand at one side. Care should be observed during sample cutting to distinguish carbon fiber orientation. In installing FG5 grids, it was instructed to face the sprayed quartzite sand upwards. For testing, the cyclic loading was applied along carbon fiber filaments (CMD). 2.3. Preparation of double-layered specimen In this study, test specimens with 150 mm diameter and 80 mm thickness were prepared using the superpave gyratory compactor. A target air void of 7 percent was used for both upper and lower layers. The preparation primarily includes four steps: (1) compaction of lower layer, (2) application of tack coat, (3) installation of grid, and (4) compaction of upper layer. Fig. 2 shows the schematic diagram of the compacted double-layered specimen. Initially, the lower layer was compacted at a 50 mm thickness within a compaction temperature of 140 °C and 160 °C for dense grade and PSMA mix, respectively. Once the lower layer was compacted, it was removed from the mold and allowed to cool at ambient room temperature for a minimum of 12 h. Tack coating then proceeded with an application rate of 0.45 L/m2. A drying period of at most 4 h was observed after tack coat application, a gentle touch on the tack coated surface was also initialized to validate dryness. Once drying was achieved, the installation of fiber grid materials followed by observing proper installation as described in previous sections. To facilitate proper cutting on specimens for OT test, a marker was used to indicate grid direction. Finally, the bottom layer with tack coat and fiber grid was inserted back into a preheated mold for upper layer compaction. The upper layer was compacted at a thickness of 30 mm producing an overall specimen thickness of 80 mm. Moreover, this research study prepared a minimum of 3 replicates for every sample configuration. 3. Performance evaluation 3.1. Overlay Tester (OT) test: cyclic fatigue loading The overlay tester has shown to properly characterize asphalt mixes based on fatigue cracking resistance potential [14,20]. The machine primarily stimulates a horizontal cyclic movement using two platens, one is fixed while the other is movable. This movement stimulates the crack opening and closing behavior on the specimen that generates the reflective cracking phenomenon. Fig. 3 shows the schematic diagram and actual test setup in this

experiment. To obtain fatigue cracking resistance on control and reinforced asphalt samples, the OT machine used a cyclic triangular displacement controlled waveform as shown in Fig. 3c. The maximum opening displacement was 0.63 mm under the loading rate of 0.1 Hz. Furthermore, the test was conducted at a temperature of 24 ± 1 °C monitored by two thermocouple wires inside the OT machine’s temperature controlled chamber. 3.1.1. OT specimen preparation Following a similar study by Walubita [16] on characterizing reinforced hot mix asphalt (HMA) samples using the OT machine, the specimen geometry used in this experiment introduced slight revisions. Fig. 4 portrays the steps in OT specimen preparation. From the compacted double-layered specimen, samples were cut to achieve 75 mm width along the reinforcing grid direction. Moreover, to obtain a thickness of 60 mm, the lower layer was further cut by 20 mm; resulting to having the fiber grid reinforcement situated equally between upper and lower layer, as shown in Fig. 4b. Furthermore, a depth of 12.5 mm notch was made to simulate cracking at midpoint resulting to an effective specimen thickness of 47.5 mm. Aside from obtaining proper sample thickness, it was necessary to cut the lower layer to obtain a smooth surface for gluing onto the OT Plates. In this experiment, the glue used was a 2 part epoxy with an adhesive tensile shear strength of 15.5 N/mm2. In addition, OT plates were spaced 2 mm apart by using a steel spacer. The glued specimen was allowed to cure for a minimum of 12 h, pressed with a 4.5 kg mass as shown in Fig. 4e. After curing, preconditioning was observed for 4 h at 24 °C, observing OT specimen temperature with a digital LCD non-contact laser infrared thermometer gun. 3.1.2. Characterization of fatigue cracking resistance 3.1.2.1. Based on number of cycles to failure. The Overlay Test procedure was conducted mainly following the test procedure Tex-248F [21]. The parameter of interest from the cyclic loading test is simply the number of load cycles to crack failure that characterizes and quantifies the HMA’s crack resistance potential. In general, the higher the number of load cycles to crack failure, the better the sample configuration is in terms of fatigue cracking resistance. From the test procedure, the number of cycles to crack failure is during 93% reduction from peak load. Given the test conditions of this experiment, it was however observed that this reduction took extensive time (>4 h) to obtain. After several dummy test, and observing good coefficient of variation on test results with substantive cycles for screening, the team adopted a 75% reduction from peak load instead. In further validating the obtained test results in terms of variability, the coefficient of variation (COV) was obtained on every sample configuration using following equation:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n    2 1u 1 X COV ¼  ¼  t xi  x x x n  1 i¼1 s

Upper layer (30 mm) Fiber grid material Tack coat

Lower layer (50 mm)

Fig. 2. Schematic diagram showing parts of the double-layered specimen.

ð1Þ 

where s is the standard deviation, x is the arithmetic mean of n terms. It was required that a minimum of 3 replicates should obtain a COV of less than 30%. Otherwise, a retest on a new specimen shall be conducted until the threshold is met [22]. 3.1.2.2. Based on critical fracture energy and crack rate index. It was observed that samples tested under monotonic loading from the OT machine displays two stages of cracking including crack initiation, and crack propagation [4,20,22] Crack initiation is defined as the cracking phenomenon before obtaining peak load of first cycle. While crack propagation is the increasing cracking damage on the specimen from the subsequent fatigue loading. Based on the report

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

Asphalt Concrete

Fiber grid

OT plates

Notch

Displacement (mm)

b.

a.

Load Cell Moving platen

Fixed platen

c.

Fig. 3. Showing (a) actual test setup, (b) schematic diagram of OT test, and the (c) cyclic triangular displacement controlled waveform.

Discarded parts 30 mm 60 mm

30 mm 20 mm

a.

75 mm

b.

Discarded part

150 mm

75 mm

c.

4.5 kg

e.

d.

Fig. 4. Showing (a) specimen at 75 mm width, (b) achieving 60 mm depth, (c) final OT specimen size, (d) curing epoxy with 4.5 kg mass, and (e) preconditioning at test temperature.

of Garcia [20], it showed possibility to delineate the performance of asphalt mixtures based on these two stages of cracking. The proposed alternative parameter in defining the stages of cracking includes Critical Fracture Energy (Gc) for crack initiation, and Crack Rate Index (Ci) for crack propagation. In computing critical fracture energy, the hysteresis loop generated from the relationship of load versus displacement was used. As shown in Fig. 5a, the area bounded by the hysteresis loop, initial zero load, and recorded maximum peak load was used to obtain critical fracture energy by using Eq. (2). In general, the higher value of Gc represents the higher energy requirement to initiate the first crack.

1 Gc ¼ tb

Z

x2

f ðxÞdx

ð2Þ

x1

where Gc is the critical fracture energy of the first loading cycle measured in J/m2, t is the effective thickness of the OT specimen, b is the width, f(x) is the curve function bounded by limits x1 and x2. In this study, MATLAB’s curve fitting tool was utilized to obtain the curve function using a polynomial regression equation with an R-square minimum of 0.95.

For crack rate index, the power regression equation obtained from the load reduction curve was utilized. As portrayed in Fig. 5b, the recorded load reduction was normalized by the maximum peak load of the first cycle. In doing so, the constant associated with the regression equation is nearly equal to unity, while the absolute value of the power term would define the crack rate index. The greater the value of crack rate index the faster is the crack propagation. 3.2. Pavement performance year analysis based on reflection cracking rate With records showing frequent reoccurrence of reflection cracking distresses on rehabilitated pavements, the study on reflection cracking rate (RCR) has been an interesting topic as a complementary data interpretation on the fracture properties obtained from the OT machine [23–26]. The analysis used in this research was developed using Microsoft Excel macro following the systematic procedure laid out by Hu [23,24], Zhou [13,25], and the MEPDG of NCHRP [27]. The principal crack growth rate model is defined based on Paris’ Law fracture mechanics as shown in Eq. (3).

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

Peak load

Crack rate index, Ci

Measured area for Gc Hysteresis loop for 1st cycle

a.

b.

Fig. 5. Illustration of (a) area bounded by hysteresis loop in measuring critical fracture energy, and (b) normalized load reduction curve in obtaining crack rate index.

dc ¼ AðDK Þn dN

ð3Þ

where c is the crack length, N is the number of load cycles, A and n are fracture properties of the material based on the results of OT test, and DK is the stress intensity factor (SIF) amplitude. Moreover, with three loading types including bending, shearing, and thermal loading on an asphalt overlay pavement, the reflection crack propagation model equation, is further defined as shown in equation (4). n

n

DC ¼ k1 AðSIF b Þ DN þ k2 AðSIF s Þ DN þ k3 AðSIF th Þ

n

ð4Þ

where DC is the crack length increment influenced by stress intensity factors due to bending (SIFb), shearing (SIFs), and thermal loading (SIFth), DN is the daily load repetitions for 80 kN ESALs, while k1, k2, and k3 are the calibration factors. Obtaining fracture properties A and n follows the work of Zhou [13]. By defining the relationship of crack length and stress intensity factor (SIF), a power regression equation curve was used to acquire fracture properties. Consequently, using different asphalt mixes would define different fracture property values. In this study, majority of interest is placed on gap-graded mix due to its common usage as an asphalt concrete material used by Korea Expressway Corporation [7]. In addition, with the formulation of numerous fracture properties on gap-graded mix, a relationship as a function of OT cycles was observed as shown in Eqs. (5) and (6).

A ¼ 0:0044ðcyclesÞ

1:91

ð5Þ

n ¼ 0:524lnðcyclesÞ þ 2:047

ð6Þ

The computation of SIF regression equations was obtained from the development of an analysis tool called SA-CrackPro [28]. The two-dimensional analysis tool utilizes thin-layer elements for simulating pavement layer contact condition and load transfer efficiency at joints and cracks. It is capable of automatic remeshing to accurately model crack propagation. The study of Zhou [25] further showed the results of SA-CrackPro on multiple SIF (shearing and bending) regression equations based on more than millions of runs. As shown in equation (7), the polynomial expression for SIFb and SIFs share the same expression but different values for Ka, Kb, Kc, Kd, and Ke.

"

SIF bending=shearing ¼ K a K b



c H1

3

 þ Kc

c H1

2

 þ Kd

c H1

#

 þ Ke

ð7Þ

where c is the crack length, H1 is the asphalt overlay thickness. For Ka, Kb, Kc, Kd, and Ke, due to complexity and extensive numerical presentation, the values used can be found in the literature [25]. In comparison, the SIFth was represented using a viscoelastic model involving thermal stress at the middle of crack spacing. The established relationship is presented similar to equation (7) with thermal

stress as a multiplier, also having different values for Ka, Kb, Kc, Kd, and Ke. In addition, the reflection cracking rate was portrayed in an empirical sigmoidal model as shown in Eq. (8). Knowing the failure RCR criteria for a pavement (e.g., RCR = 50%), the performance year of a HMA mix overlay is defined as the number of months until reaching failure criteria.

RCR ¼

100 1 þ eC 1 logðDC=tp Þ

ð8Þ

where RCR is the reflection crack rate percentage, C1 as the relationship between fatigue distress versus damage (equal to 4.2) [26]; and tp as the overlay thickness. The pavement performance year analysis mainly requires input data including climate, traffic, material properties, and pavement geometry, as shown in Table 3. Dynamic modulus test was performed in accordance with AASHTO T 342 [29], while the general formula used for generating the master curve is based on MEPDG

Table 3 Summary for pavement performance year analysis input data. Asphalt Overlay Information

Input data

Type Analysis Period RCR Termination Criteria Traffic Data ADT 80kN ESALs Operation speed Climate Data (10 yr. period) Material Properties HMA Overlay Thickness Mix Type Poisson’s Ratio Load Cycles Coefficient of Thermal Expansion (CTE) Dynamic Modulus Existing PCC Layer Thickness Mix Type Poisson’s Ratio Coefficient of Thermal Expansion (CTE) Crack Spacing Modulus Load Transfer Efficiency (LTE) Base Layer Thickness Material Type Poisson’s Ratio Typical Modulus Subgrade Poisson’s Ratio Typical Modulus

AC/PCC 20 yrs. 50% 10,000 veh/day 1.3 million 60 kph +38 °C to 5 °C

50 mm SMA D, PG76-22 binder 0.35 236, 677, and 1105 24.3  10–6 mm/mm/°C (Data by AASHTO T 342–11) 200 mm JPCP 0.15 9.9  10–6 mm/mm/°C 4.5 m 27,560 MPa 50% 200 mm Granular Base 0.35 1035 MPa 0.4 70 MPa

A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

[27] as shown in Eq. (9), using Arrhenius equation for reduced frequency.

logjE j ¼ d þ

ðjE jmax  dÞ D Ea 1 1 1 þ ebþcflogðxÞþ19:14714½ðT ÞðT r Þg

n X

Ei eðt=si Þ

4.1. Result on fatigue cracking resistance

ð10Þ

i¼1

where Erel is the relaxation modulus in MPa, Ee is the equilibrium modulus, Ei as the relaxation strengths in MPa, with si as the relaxation times in seconds, and n as the total number of Maxwell elements. Utilizing Eq. (10) in the Boltzmann superposition principle [27] will give the thermal stress as shown in Eq. (11).

rðtÞ ¼

n X

Eðt  nj ÞDej

4. Test results and discussions

ð9Þ

where |E*| is the dynamic modulus in MPa, |E*|max as the maximum limiting modulus estimated from mixture volumetric properties using the Hirsch model [30] in MPa; d, b, c, and DEa are fitting parameters, x is the loading frequency in Hz, while T and Tr are the test temperature and reference temperature in °K, respectively. Due to the long thermal loading times experienced by an asphalt overlay, a hybrid approach was used for computing SIFth that included the thermal stress at the far field. To compute for the thermal stress, the relaxation modulus of the HMA mix was utilized based on Maxwell’s generalized model [31] by characterizing the dynamic modulus in a Prony series, as portrayed in Eq. (10).

Erel ðtÞ ¼ Ee þ

7

ð11Þ

j¼1

where r(t) is thermal stress at time t, E(t  n) is the relaxation modulus at time t-n, and De is the change in strain at time n previous to t (=CTE (T(n)-T0). T(n) and T0 are pavement temperature at time n and pavement temperature when r = 0, respectively. CTE is the coefficient of thermal expansion. The development of pavement performance year analysis was limited on the analyzation of a single type HMA overlay layer, specifically for gap-graded mix on a plain concrete pavement. The program intended to create a feasible conversion on the mechanistic OT test in computing the RCR of a pavement with fracture properties influenced by a fiber grid reinforcement. With this reason, the author’s team decided to conduct the analysis on three sample configurations namely for sample #1, #6, and #7 only.

4.1.1. Based on number of cycles to failure The OT test result from the cyclic fatigue loading is depicted in Fig. 6. As portrayed, sample #7 showed the highest fatigue cracking resistance based on the number of cycles to failure that may have been attributed to the presence of carbon fiber grid reinforcement. As previously discussed, FG5 grid had higher tensile strength along CMD which influenced fatigue cracking resistance greatly. Compared to its counterpart having glass fiber grid reinforcement, FG4 grid reinforced PSMA mix only obtain second highest with 677 average cycles to failure. Furthermore, using carbon fiber grid reinforcement increases the number of cycles to failure by more than 4 times compared to control sample. The influence of the physical property of a fiber grid material is further portrayed on the usage of FG3 and FG4 grid, due to similar material tensile strength, OT cycles did not show a substantial difference. Validating the mixture used in this experiment, Li [32] showed that the OT pass-fail screening criteria for gap-graded mix should have a minimum of 300 cycles. Although the control sample, PSMA mix, obtained only 236 cycles, it is important to note that this failure criteria was based on 75% load drop. Further increasing the load drop percentage to 93% would most likely permit the PSMA mixture obtaining the screening criteria of 300 cycles. This goes to show that the PSMA mixture used in this experiment is within normal performance condition. In addition, the influence of using a PSMA lower layer mixture is depicted by comparing samples #5 and #6. From the OT test result, using a gap-graded mix potentially increases fatigue cracking resistance by 24%. From the OT test, an unexpected result was observed among the fiber grid reinforced samples. Sample #3 with FG2 grid exhibited the poorest fatigue cracking resistance with 40 cycles to failure. The low performance may be attributed to the material property selected as the grid’s film backing. It was reported that the PE film used in FG2 grid typically melts at a temperature of 110 °C. Since the preparation of a double-layered specimen required the lower layer to be cooled at room temperature, this may have hindered the film from breaking down during compaction. Although the emulsified tack coat used had high ISS value, the presence of a film between layers can prevent proper aggregate interlocking that may

Fig. 6. Showing OT cycle average for different sample no. with corresponding COV.

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

have caused a weak interlayer structure. Moreover, the aperture size along MD for FG2 grid had 20 mm. This was not conducive during OT test since only an estimated 3 ribs were included on the OT specimen after cutting into a size of 75 mm width. The fatigue cracking resistance for FG1 grid reinforced samples showed only a slight improvement compared to control sample by 12%. Although there was a presence of a reinforcing material, the lack of tenacity and complementary physical attributes, such as film or coating, made the fiber filaments on FG1 grid less durable. The fiber filaments showed that just by a slight force would have made the rib formation break easily. It has been recorded that fatigue dynamic loading test usually shows undesirable repeatability and variability [21]. To assure that test results obtain an accurate representation of the material property, the COV of 30% was used as a threshold. As portrayed in Fig. 6, all three replicates used to represent fatigue cracking resistance were within the COV criteria. 4.1.2. Based on critical fracture energy and crack rate index The results on the computation of critical fracture energy and crack rate index are presented in Table 4. In relation to background literature, results presented in this study showed agreement that the use of fiber grid reinforcement material did not affect much

Table 4 Test summary results of critical fracture energy and crack rate index. Sample

Parameters

Critical Fracture Energy, Gc (J/ m2)

Crack Rate Index, Ci

#1

Average Std Dev COV Average Std Dev COV Average Std Dev COV Average Std Dev COV Average Std Dev COV Average Std Dev COV Average Std Dev COV

256 0.034 13.1% 245 0.044 18.1% 134 0.031 22.9% 363 0.046 12.7% 304 0.023 7.6% 311 0.014 4.6% 321 0.031 9.7%

0.257 0.007 2.6% 0.195 0.041 20.9% 0.329 0.004 1.3% 0.178 0.004 2.3% 0.190 0.012 6.1% 0.156 0.008 5.4% 0.147 0.009 5.9%

#2

#3

#4

#5

#6

#7

during crack initiation but rather during crack progression. From Table 4, comparing different lower layer mixes showed critical fracture energy having no conclusive trend. The result portrayed an inconsistent quantification that may or may not have been influenced by the fiber grid material. Moreover, the crack rate index of sample #7 showed the lowest value, indicating a slow cracking phenomenon. This result validates the previous finding that using FG5 grid is observed to have the best fatigue cracking resistance among all sample configuration. As shown in Fig. 7, the crack rate index also showed a good correlation with the number of cycles to failure. With this observation and while having a much lower coefficient of variation, using the concept of crack rate index may prove viable as a substitute on data interpretation for fatigue cracking resistance.

4.1.3. Statistical analysis using Tukey’s HSD test The Tukey’s HSD (Honest Significant Difference) test was utilized as a statistical tool to compare the results obtained from OT load cycles and crack rate index test. This statistical analysis will help determine if each sample configuration is mutually statistically significant. The critical value of q used in this analysis was for a = 0.05. The results are presented in Table 5. From the statistical data, 4 groupings were observed from the 7 sample configurations on both types of fatigue cracking resistance measurement. Groupings were named in alphabetical order, with Group A as the highest fatigue cracking resistance and Group D as the poorest performing reinforced material. Although crack rate index test presented identical material rankings compared to OT load cycles, the Tukey’s HSD analysis portrayed different groupings. Based on Table 5, it is observed that the OT load cycle delineates fiber grid reinforcing materials reasonably than crack rate index test. From the groupings, the material superiority of FG5 grid was emphasized, while FG2 grid ranks as the poorest and falls in the lowest group. Under OT load cycle results, FG4 and FG3 grid were not significantly different. In addition, the control sample and reinforced FG1 grid were within the same group ranking. This presentation is in contrast to the groupings found in crack rate index test, such that control sample was significantly different from any fiber grid reinforced material.

4.1.4. Effective benefit ratio In further ranking fiber grid materials, the effectiveness based on traffic benefit ratio (TBR) concept was utilized. TBR concept defines the improvement ratio of a reinforced sample from the unreinforced sample based on load cycles [33]. In this study, the effectiveness benefit ratio was applied to fatigue cracking resistance results only. The computation of the effectiveness benefit ratio (EBR) is shown in Equation (12). Furthermore, the summary of results is presented in Table 6.

EBRð%Þ ¼

Fig. 7. Showing relationship of OT cycle and crack rate index with good r-squared value.

TBRr  TBRu  100 TBRu

ð12Þ

where TBRr is the TRB for reinforced sample, and TRBu is the TBR for unreinforced sample. Based on the conditions set by this experiment, the use of fiber grid reinforcement clearly shows a beneficial influence. With 5 out of 6 fiber grid reinforced samples portraying 11% to more than 100% improvement based on effectiveness. Using FG5 grid (sample #7) showed the highest TBR value of 4.86, followed by FG4 grid (sample #6) with TBR value of 2.87. Meanwhile, for sample #3 using FG2 grid showed a negative effectiveness benefit ratio implying the importance of correct installation guide and proper grid aperture size.

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873 Table 5 Statistical comparison for all sample configurations. Sample

Grid

Ranking

#1 #2 #3 #4 #5 #6 #7

none FG-1 FG-2 FG-3 FG-4 FG-4 FG-5

6 5 7 3 4 2 1

Table 6 Test result summary on the effectiveness benefit ratio. Sample #

OT Cycles

TBR

EBR

Ranking

7 6 4 5 2 1 3

1105 677 573 546 263 236 70

4.68 2.87 2.43 2.31 1.11 1.00 0.29

>100% >100% >100% >100% 11.0% 0.0% 71.0%

1 2 3 4 5 6 7

4.2. Pavement performance year calculation The developed pavement performance year analysis program is capable of automatically stimulating the reflection cracking rate model from various input data presented in Table 3 using Microsoft Excel macro. The dynamic modulus and relaxation curve in this experiment are presented in Fig. 8. In addition to the material

Tukey’s HSD analysis OT load cycle

Crack Rate Index

Group C

Group Group Group Group

Group D Group B

C B D B

Group A Group A

property, actual climate data was used as shown in Fig. 9 in computing pavement temperature and thermal stress. The foremost output presentation of the developed program is a graph showing the behavior of reflection cracking rate of the considered asphalt overlay pavement as shown in Fig. 9a. From the analysis, samples #1 (236 cycles), #6 (677 cycles), and #7 (1105 cycles) obtained a pavement performance year of 2.6, 5.6, and 7.7 years respectively. These pavement performance year values would represent the duration of acceptable service life wherein RCR is less than 50%. The use of fiber grid reinforcing material, specifically FG4 and FG5 grid shows an increase in performance year by 2.15 and 2.96 times, respectively. By using the actual mixture dynamic modulus, climate data, vehicular traffic, and pavement thickness, the developed program gives a more holistic approach towards defining the improvement caused by the fiber grid reinforcing material. As obtained, the increase in pavement performance year is in comparison to the reviewed literature range of 1.1 to 6.0 [4,6,35].

Fig. 8. Showing material property analysis for (a) dynamic modulus master curve, and (b) conversion to relaxation modulus using Prony series.

Fig. 9. Climate data used for pavement performance year analysis.

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A.B. Tam et al. / Construction and Building Materials 239 (2020) 117873

Fig. 10. Pavement performance year analysis graph based on (a) original sigmoidal model, and (b) improved full s-shape graph model.

Due to its incapability of portraying a full graph, Hu [34] proposed a new reflection crack rate empirical model as shown in equation (13). This new model portrays a full s-shape graph which ideally shows a better representation on field cases as shown in Fig. 10b. With the improved data presentation, comparison with other overlay design approaches such as found in the study of Zofka [12] and MEPDG [27] would be plausible in future works.

RCR ¼

100 euðq=mÞ

b

ð13Þ

where q is the curve width directly determined based on the crack length incremental calculation in terms of month, m is the month number, b is the curve slope, and u is a dimensionless constant with a value of 0.693147. 5. Conclusion In this research, the evaluation of fatigue cracking resistance on reinforced and unreinforced double-layered asphalt concrete specimen was conducted by inducing a cyclic triangular displacement controlled loading using the OT machine. In interpreting RCR based on fracture properties, a newly developed Microsoft Excel macro program was utilized. The following key findings and conclusions were drawn from the study:
Considering the test conditions and parameters, almost all HMA samples with fiber grid reinforcement exhibited an improved performance on fatigue cracking resistance. With an emphasis on the reinforcing material’s physical property (e.g., tensile strength, coating, and aperture size), results can obtain more than a 100% improvement compared to control sample. From the number of cycles to failure, FG5 grid with carbon fiber filaments obtained highest effectiveness benefit ratio followed by FG3, FG4, FG1, and lastly FG2 grid.
PSMA mix outperforms dense graded mixes and would be the preferred mix to provide high fatigue cracking resistance. With an improvement of 24% based on the number of cycles to failure. However, PSMA is a competitively expensive mix and thus, should always be selected cautiously bearing in mind the economics and constructability aspects related to it.
At the test temperature of 24 ± 1 °C, the OT test repeatability and variability can achieve acceptable limits of COV less than 30%. From this study, it was observed that utilizing the test temperature obtains unreasonable OT load cycles for high load peak drop percentage. Therefore, it is recommended to use only 75% load peak drop following the test conditions stated in this study.


Utilizing fiber grid reinforcement did not show conclusive results in characterizing fatigue cracking resistance based on critical fracture energy. However, the concept of crack rate index proves viable as a surrogate parameter while showing a lower COV value compared to the number of cycles to failure.
The developed Microsoft Excel macro program was able to automatically portray the pavement performance year analysis with the given data inputs. The systematic method was able to follow the work of TTI and showed significant importance of using fiber grid reinforcement, with the pavement performance year benefit of 2.15–2.96 times for FG4 and FG5 grids, respectively. Overall, the result presented by the OT test showed possibility in quantifying the influence of the fiber grid reinforcing material. Not only did it showed relevance to the physical property of the reinforcing material, but it also portrayed the importance of fiber grid installation to optimize performance. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was conducted under research project Development of High-Performance Concrete Pavement Maintenance Technology to Extend Roadway Life (Project No : 18TLRP-B146707-01) funded by the Ministry of Land, Infrastructure and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KAIA). The authors would like to thank the members of research team, MOLIT and KAIA. References [1] S. Deilami, G. White. Review of reflective cracking mechanisms and mitigations for airport pavements. Presented at 28th ARRB International Conference, Brisbane, Queensland, 2018. [2] R.F. Carmichael III, M.L. Marienfeld, Synthesis and literature review of nonwoven paving fabrics performance in overlays, Transp. Res. Record: J. Transport. Res. Board 1687 (1999) 112–124, https://doi.org/10.3141/1687-13. [3] A. Zofka, M. Maliszewski, E. Zofka, M. Paliukaite˙, L. Zˇalimiene˙, Geogrid reinforcement of aspahlt pavements, Baltic J. Road Bridge Eng. 12 (2017) 181–186, https://doi.org/10.3846/bjrbe.2017.22. [4] M.L. Nguyen, J. Blanc, J.P. Kerzreho, P. Hornych, Review of glass fibre grid use for pavement reinforcement and APT experiments at IFSTTAR, Road Mater. Pavem. Des. 14 (sup1) (2013) pp. 287–308, https://doi.org/10.1080/ 14680629.2013.774763.

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